The present invention is generally related to the field of communications, and, more particularly, is related to a system and method for noise communication using noise modulation.
In many circumstances regarding communications, it is desirable that the information transmitted from one point to the next be kept secret from outside parties. For example, in commercial communications, one may wish to communicate sensitive financial information without one""s competitor being able to determine the information sent or to even be aware of the fact that a message was sent. As an alternative example, in military applications, one may wish to communicate without one""s enemy being able to intercept and decode the message sent. In pursuit of a communications approach that would meet such demands, noise signaling has been pioneered. The concept of noise signaling has had a history that, much like the broader history of spread spectrum communications of which it is a part, has been superbly documented in, for example, Simon M. K., Omura J. K., Scholtz R. A., and Levitt B. K., Spread Spectrum Communications, Vol. 1, Chapter 2, Computer Science Press, 1985.
Much of the earlier efforts in noise communications centered on the problem of generating the xe2x80x9crandomnessxe2x80x9d that would be used to disguise, mask or scramble a transmitted communication signal. This same randomness would have to be faithfully reproduced at the receiving end of the communication link in order to achieve the complementary goal of revealing, unmasking or unscrambling the received signal for the intended listener. Historically, the process of randomization has taken many forms. In addition to the familiar pseudo-random sequences used in Direct Sequence Spread Spectrum (DSSS), frequency hopping, and time hopping, inventors have exploited less familiar techniques aspiring to communication security. There are a number of approaches, for example, that scramble temporal elements of the transmitted communication signal discussed in U.S. Pat. No. 3,824,467 issued to Charles, U.S. Pat. No. 3,978,288 issued to Bruckner, et al., and U.S. Pat. No. 3,921,151 issued to Guanella.
Historically, spread spectrum communications has made use of binary pseudorandom sequences. This initial focus was motivated by the need for simplicity in implementation and control. In those earlier years, the computational power and storage capabilities of small modern computers was unanticipated. The classic example of earlier attempts at noise communication is the famous noise wheel of DeRosa and Rogoff in U.S. Pat. Nos. 2,718,638 and 4,176,316 described at considerable length in Simon M. K., Omura J. K., Scholtz R. A., and Levitt B. K., Spread Spectrum Communications, Vol. 1, Chapter 2, Computer Science Press, 1985. As one would expect from a mechanically rotating wheel, this device created a source of cyclically repetitive noise energy. To replicate randomness, Rogoff generated a radial plot of the middle digits of numbers randomly selected from the Manhattan phone directory. Later the plot was transferred to film and, once placed on the wheel, was rotated past a slot of light that intensity-modulated the light in accordance with the length of each radial slot. Information modulation was finally achieved through time-shift keying, i.e., switching between time wheels rotating at slightly different phase offsets. The system accomplished information transfer of approximately one bit per second over a distance of two hundred yards.
Another important contribution is that of Klund in U.S. Pat. No. 5,493,612. This invention uses two techniques to do the information modulation. The first can be thought of as M-ary Frequency Shift Keying (FSK) of the output from a single noise generator. It involves the transmitting of information by essentially changing the carrier frequency in accord with the data symbol by selecting from a very closely spaced set of M frequencies. Filter parameters are chosen so that bandpass filtering of the noise transmission forces the output spectrum to take on the same appearance in each case.
The second technique discussed in U.S. Pat. No. 5,493,612 includes a transmitter which uses a single carrier frequency and selects between noise generators to represent a particular data symbol. This transmitter makes use of analog waveforms which results in spectral splatter due to the discontinuities that occur when the information symbols are imposed on the noise, which this reference fails to discuss.
Another example that makes use of noise is the secret signaling system of Bitzer in U.S. Pat. No. 4,179,658 in which the basic information signal comprises a frequency modulated (FM) voice message. Through a balanced modulator the FM voice input is multiplied by an analog noise signal. Through a separate path the same noise signal is delayed then modulated onto the carrier. The two waveforms, the noise modulated FM voice and the delayed noise waveform (without information superimposed), are then linearly added, thus generating the transmitted signal. With the separate addition of an appropriate delay in the signal path at the receiver, one is able to obtain the reference noise waveform in the received transmission and, thus, demodulate the data. Schemes like this that include the reference noise waveform in the transmission are subject to intercept. In fact, the scheme just described has a very fundamental vulnerability; at just the right delay an interceptor will find that the received signal will correlate very strongly with a delayed version of itself. Additionally, it is not clear to what degree the slower variations of the information signal will affect the measurable statistics of the noise. Clearly, a very slow information signal would introduce a slow, most likely nonstationary, variation into the random noise.
Another secure communication approach is to randomize the transmitted signal by first sending it through a xe2x80x9crandomxe2x80x9d filter. The device described in U.S. Pat. No. 4,393,276 issued to Steele, for example, scrambles the signal in the frequency domain by applying a mask to the Fourier transform of the signal. Because the mask parameters are shared with the receiver, the receiver is able to invert the mask at the other end of the communication link. Also, one signal processing scheme, for example, xe2x80x9crandomizesxe2x80x9d the power level to simulate fading (U.S. Pat. No. 4,658,436 to Hill) and thus gives the transmission a more natural appearance in the environment.
In contradistinction to the approaches made above, some systems directly radiate noise to mask the existence of an information-bearing signal. Motivated by the fact that directional antennas are subject to enemy sidelobe detection, we find in U.S. Pat. No. 4,397,034 to Cox, et al., for example, an omni antenna used to radiate noise into the sidelobes of a highly directional (one degree beamwidth) antenna. With the noise signal statistically related to the information transmission in order to aid in the masking, the goal of this scheme is to prevent the detection of the information transmission.
Although examples in the journal literature are sparse, the use of noise for communications has not been totally neglected by analysts. Bello, for example, has studied a communication system in which the information-bearing signal phase modulates a noise carrier. Bello, P. A., xe2x80x9cDemodulation of a Phase-modulated Noise Carrier,xe2x80x9d IRE Transactions on Information Theory, vol. IT-7, no. 1, pp. 19-27, January 1961. In this reference, the effect of additive Gaussian noise and linear filtering on the first-order statistics of the receiver output noise and some aspects of the output signal are presented along with some simplifications relative to modeling the distortion of the output signal.
Due for the most part to the state of technology of the time, the approaches described above suffer from a number of limitations. Of particular importance is the restrictive limit on availability of noise waveforms at both transmitter and receiver. This is seen, for example, in the noise wheel of Rogoff, et al. in which the xe2x80x9crandomnessxe2x80x9d becomes a fixed part of a wheel that can only hold a small number of random variables because of its finite circumference.
Most of the approaches described in the patent literature that involve the use of true noise are analog in nature. A prototypical example is the multiplication of an information-bearing stream of signals by an analog noise reference carrier. Typically, in such an operation the imposition of information changes the statistical character of the noise. When the information is in the form of a stream of symbols, the transitions between the different symbols appear as discontinuities that give rise to spectral splatter. Such degradation is pervasive. Although one may start with pure wide-sense stationary noise waveforms, the fundamental periodicity of most modern information-bearing communication signals introduces cyclostationary disturbances to the noise. For communication designers interested in covertness, this introduces spectral lines and other features which, unfortunately, waste energy and comprise exactly those features that would be of interest to and could be exploited by an unfriendly interceptor presence.
The present invention provides a noise communication system which comprises a transmitter and a receiver. Both the transmitter and receiver include, for example, programmable processor based circuits which are programmed to perform noise modulation according to the various embodiments of the present invention, although dedicated logical circuits may be employed as well.
The transmitter includes logic to index through at least two noise records each of which comprise a series of randomly generated samples. Specifically, each of the noise records is divided into noise segments upon which the indexing function is applied. At any given moment, the indexing function identifies a current noise segment for each noise record.
The transmitter also includes logic to modulate a predefined base signal which may be, for example, a voice signal, data signal, or other information source into a noise signal. Each noise record is a source of noise samples for representing a particular symbol of the base signal symbol alphabet. In modulating the predefined base signal, the transmitter replaces the respective symbols of the base signal with the current noise segments from the respective noise records, thereby generating a noise signal in which neither the symbols nor the transmissions between the symbols can be discerned.
The noise signal is transmitted across a communications channel to the receiver which includes logic to demodulate the noise signal into the base signal. The demodulation employs a number of correlators that equals the number of symbols in the base signal and the number of noise records employed at the transmitter. The receiver includes logic to index through the noise records in a similar manner to the indexing performed in the transmitter to produce the same current noise segments for each noise record. Each correlator performs a multiplication function between a current noise segment of samples from the noise record assigned to the correlator and the received segments of samples of the noise signal which reveals a peak output when the segments match. The base signal is recreated by incorporating the symbol corresponding to the noise record that the correlator indicates is the best match to the segment of the received signal.
The present invention can also be viewed as providing a method for modulating a predefined base data signal comprising a stream of symbols from a predefined alphabet of symbols into a noise signal, comprising the steps of: indexing through a plurality of predefined noise segments of at least two noise records, each noise record corresponding to a symbol from the predefined alphabet of symbols; and, modulating the predefined base signal into the noise signal by replacing each the symbols of the predefined base signal with one of the predefined noise segments from the noise record corresponding to each respective symbol.
The present invention may also be viewed as a method for demodulating a predefined noise signal comprising a stream of noise signal segments into a base signal comprising a stream of symbols from a predefined alphabet of symbols, the method comprising the steps of: indexing through a plurality of noise record segments of at least two predefined noise records, each noise record corresponding to symbol from the predefined alphabet of symbols; and demodulating the predefined noise signal by correlating each of the noise signal segments with the indexed noise record segments and determining a maximum correlation value for each of the noise signal segments, the correlation value corresponding to one of the symbols of the predefined alphabet.
The noise communication system and method of the present invention feature significant advantages in that the noise signal generated appears to be actual noise without any periodicities or spectral splatter, making the signal virtually immune to interception and decoding by unauthorized receivers without the noise records employed by the transmitter. Each noise signal can be transmitted in a single frequency band, or multiple noise signals or channels can be transmitted using the same frequency band. In addition, multiple channels may be transmitted using multiple adjacent frequency bands, thus creating an appearance of a spread spectrum signal of even greater bandwidth than the single channel spread spectrum signal. In addition, the base signal may be distributed among multiple channels, thereby increasing the speed of the communication.
The present invention is characterized by, but is not limited to, other advantages such as simplicity of design, user friendliness, robust and reliable operation, efficient operation, and easy implementation for mass commercial production.
Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention.